While thumbs allow humans to both wield an axe and focus a microscope, they are a hindrance for some primates, including spider monkeys. These are agile climbers that travel through the canopy using long, rapid swings. With no thumbs, their hands act like hooks, allowing them to swing and grasp in rapid, repeated strokes.
How did thumbs develop?
What most likely started as a random mutation with a shortened thumb would have been selected and become normal. And while no thumbs helps these monkeys move, it does prevent grooming. To replace this important bonding activity, the animals cuddle to reinforce their social links.
Main image: Black headed Spider Monkey © Manoj Shah/Getty Images